The present invention relates generally to catheters for introduction into a patient's vascular system and methods of constructing such catheters. In particular, the present invention relates to an atherectomy catheter having a plurality of concentrically arranged shafts, the shafts having a multi layer torsionally reinforced construction for improved torsional stiffness and positionability, low profile and improved manufacturability.
Arteriosclerosis is a well-known disease of the vascular system in which fatty deposits, or atheroma, are deposited on the intimal lining of the patient's blood vessels. This can result in stenotic regions which may partially or completely occlude the vessel, inhibiting blood flow through the vessel. Arteriosclerosis may produce a variety of health consequences, including angina, hypertension, myocardial infarction, and strokes.
Various devices have been developed for treatment of arteriosclerosis. One such device which has shown promising results is the atherectomy catheter. Such catheters typically comprise an elongated, flexible body having a device on its distal end designed to sever stenotic material from the vessel wall. A balloon is frequently mounted at the distal end to assist in positioning directional severing device against the vessel wall. The catheter is inserted into an artery and advanced through the artery to the desired treatment site with the balloon in a deflated configuration. Rotational positioning is usually required in order to properly position the severing device adjacent the stenotic material to be excised. Such rotation is usually accomplished by exerting torque on the proximal end of the catheter so as to twist the distal end to the desired position. An inflation fluid is then supplied to the balloon through a lumen in the catheter body so as to expand the balloon and position the severing device against the vessel wall.
The severing devices which have been used in such atherectomy catheters have taken various forms. Of particular interest to the present invention is the use of a cylindrical or helical cutting blade which is rotatably mounted in a housing at the distal end of a catheter body. A drive shaft extends from the proximal end of the catheter body through a lumen to the distal end where it is coupled to the cutting blade. A drive motor at the proximal end rotates the drive shaft so as to turn the blade. Usually, the blade will be partially exposed through an opening on one side of the housing. Tissue may be positioned in the opening and the blade advanced against the tissue as it is rotated so as to sever the tissue. The opening is rotationally positioned within the vessel by exerting torque on the proximal end of the catheter body, so as to turn the housing until the opening is properly positioned adjacent the stenotic material to be severed. A balloon is typically mounted on the housing opposite the opening, such that the opening can be positioned against the vessel wall when the balloon is expanded.
Atherectomy catheters must be constructed in such a way that they have an appropriate degree of flexibility as well as a low profile (i.e. small diameter) so as to be positionable in a blood vessel. At the same time, such catheters must have suitable torsional stiffness to facilitate rotational positioning of the opening in the housing which exposes cutting device by exerting torque on the proximal end. Such catheters will usually have multiple lumens, including a lumen for introducing an inflation fluid to the balloon, a guide wire lumen which receiving a movable guide wire as the catheter is advanced to navigate the vessel, as well as a lumen through which the drive shaft of the cutting blade may be rotationally disposed. The achievement of a catheter with the desired flexibility, torsional rigidity, small profile and durability in a way which is durable and manufacturable at low cost is a challenge.
In particular, the configuration of the lumens in known catheters has resulted in a somewhat complex structure which is costly to manufacture. For example, the inflation lumen, drive shaft lumen and guide wire lumen in known devices are often formed as separate lumens and arranged in parallel along the length of the catheter body.
In addition, known manufacturing techniques for producing catheters having enhanced torsional rigidity have suffered from certain drawbacks. One known method for producing a catheter body with enhanced torsional rigidity involves imbedding a pattern of a reinforcing material, such as braided wire or fibers, into the wall of a tubular polymer shaft. Typically, the reinforcing material is positioned over a mandril having an inside diameter of the desired finished dimension. A second layer of polymeric material is extruded over the reinforcing material so that the reinforcement is encapsulated within the resulting polymeric wall of the catheter body.
An improvement on this method for imbedding reinforcement material in the wall of the catheter body is described in U.S. Pat. No. 4,764,324 to Burnham (the '324 patent), the complete disclosure of which is incorporated herein by reference. The improved method involves placing the reinforcing material, such as a wire braid or helical wrap, over a polymer tube and heating the tube while simultaneously applying axial tension to the reinforcement. This causes the reinforcement to penetrate beneath the surface of the polymer tube, and the penetration depth is controlled by controlling the temperature of the catheter body and the tension exerted on the reinforcing material. Once the reinforcing material is submerged to the proper depth in the tube, the waffled outer contour of the tube caused by the impregnation of the reinforcing material is smoothed to the desired dimension by passing the substrate through a die.
While the extrusion technique described in the '324 patent may have certain advantages, the method suffers from difficulty in controlling the dimensions of the catheter wall and the depth to which the reinforcing material penetrates.
Where a rotatable cutting blade is utilized in an atherectomy catheter as described above, the lumen through which the drive shaft or cable is disposed should have a suitable degree of lubricity to permit the shaft to rotate with minimal friction even when the catheter is configured in a tortuous path in the vessel. One known technique for providing a low friction surface on the catheter lumen wall is described in U.S. Pat. No. 4,898,591, the complete disclosure of which is incorporated herein by reference. A lubricous hydrogel coating of a biocompatible material such as a copolymer of polyurethane and polyvinyl pyrrolidone is applied to the surface of the catheter lumen by flushing the hydrogel solution through the lumen of the catheter, dipping the catheter in a bath of the solution or spraying the solution onto the desired surfaces, followed by drying and curing in an oven. This technique is labor-intensive and increases manufacturing costs.
An improved atherectomy catheter is therefore desired which is highly flexible for positioning in the vascular system, while having high torsional rigidity for rotational positionability. The catheter should have a low profile, and preferably be round in cross section to facilitate positioning within the confines of a blood vessel. Particularly desirable is a catheter which is manufacturable using low-cost, automated processes, capable of repeatably and accurately controlling catheter dimensions and quality. In particular, the catheter should have a torsionally reinforced body, capable of including at least three lumens, one through which an inflation fluid may be introduced, one in which a rotatable drive shaft or cable may be disposed, and a third through which a movable guide wire may be inserted. The drive shaft lumen should have a low-friction surface in contact with the drive shaft to facilitate low-friction rotation of the drive shaft with the catheter in various longitudinal configurations. In addition, the inflation lumen should be comprised so as to be very durable, resisting puncturing during manipulation.